7. PROJECT SUMMARY/ABSTRACT Structural hip deformities, including femoroacetabular impingement (FAI) and developmental dysplasia of the hip (DDH), are orthopaedic conditions that alter the coverage of the femoral head. Specifically, FAI leads to over-coverage and DDH leads to under-coverage of the femoral head, and both are common causes of dysfunction. Without treatment, the altered geometries substantially predispose patients to degenerative joint changes due to increased cartilage loading, including hip osteoarthritis (OA), by upwards of 4.3 fold. Although altered cartilage loading is a primary instigator of the onset of OA, it cannot be measured in vivo. As a result, computational models (e.g. finite or discrete element analysis) are the most widely used surrogates for cartilage contact pressure estimations. However, these models have two primary limitations that impede their clinical utility: 1) the computational burden often remains too large to solve highly dynamic activities and 2) they do not simultaneously integrate the roles of complex patient-specific joint geometry, joint kinematics, and muscle function. The first goal of this proposal is to develop and validate a subject-specific musculoskeletal model that is capable of estimating dynamic cartilage loading contacts in patients with FAI and DDH. To mitigate pain and prevent the development of degenerative secondary pain conditions in patients with DDH, both surgical and nonsurgical approaches exist, yet both have varied success. Surgical intervention aims to provide more uniform distribution of hip loads by correcting bony abnormalities to appear more like healthy (asymptomatic hips) and is specific to pathology type. Although effective at improving short-term pain and function, long-term functional deficits remain (e.g. pain, limping, reduced joint space) that lead to the eventual development of end-stage hip OA and/or total hip arthroplasty. Contrarily, rehabilitation involves general muscle strengthening, movement retraining, and improvements of range of motion. However, it remains not well defined, lacks validation, and is not specific to pathology type. Given the difference in coverage of the femoral head between FAI and DDH, it is likely that cartilage contact pressures are affected differently with the same intervention, yet this effect has not been explored. The second goal of this proposal will be to establish the effects of simulated surgical and non-surgical intervention on hip joint cartilage contact pressure in patients with FAI and DDH. This proposal will be the first to develop a musculoskeletal model capable of estimating hip joint cartilage loading by simultaneously modeling the effect of subject-specific geometry, kinematics, and muscle function (Aim 1) and to assess the effects of simulated intervention on hip joint cartilage loading patterns (Aim 2). Understanding how cartilage contact mechanics change with intervention, and establishing differences specific to pathology types (FAI vs DDH), will inform future treatment strategies that are tailored to the specific pathology in an effort to improve pain and function.